Worldwide interest in reducing pollution and improving fuel economy has driven a variety of government regulations demanding ever-improving performance from truck and automobile engines. Automotive companies have achieved past goals for emissions and fuel economy using various technologies such as air-to-air aftercooling, high compression piston ratios, lowering engine mechanical friction, lowering engine pumping losses, advanced exhaust gas recirculation (“EGR”) and other such aftertreatment systems, among others. Also, combustion efficiency has been increased by use of high injection pressure fuel systems, fine-tuning of injector nozzle configuration, piston bowl, air motion (“swirl”), and the amount of volume above the rim of the piston producing a “squish.” The use of various aftertreatment systems, which remove emissions from engine exhaust before releasing from the vehicle, like Diesel Oxidation Catalyst (“DOC”), has further reduced exhaust pollutants.
A more radical strategy for reducing emissions and improving fuel economy was to replace internal combustion engines with electric motors. Electric motors have a tremendous advantage over combustion engines, especially in city driving, because they can conveniently recapture kinetic energy of the vehicle by connecting the brakes to an electric generator.
In past years, a trickle of electric cars powered by batteries were produced. But because the batteries were heavy and expensive, the cars didn't drive far enough between recharges to satisfy consumers. Even government incentives failed to produce a substantial number of sales of electric vehicles.
A promising solution to the battery problem in electric vehicles is the “hybrid vehicle.” Hybrid vehicles employ a hybrid power train, including both a combustion engine and an electric motor. Consequently, gasoline, diesel fuel, or other such fuels can still be used as an efficient means of storing the energy needed to give the vehicle satisfactory operating range, while still realizing many of the advantages of an electric motor, including recapture of braking energy.
Hybrid vehicles fall generally into one of two general categories: series and parallel hybrids, as shown in FIGS. 1A and 1B. FIG. 1A shows a block diagram for a typical series hybrid, indicated generally at 120. The engine 122 is coupled to a generator 124, and an aftertreatment system 139. The generator 124 supplies power to a motor/generator 128, which, in turn, drives the wheels 130 (possibly through a differential 129). An energy storage system 126, comprising one or more batteries, receives power from the motor/generator 128 during braking, and also directly from the generator 124 as may be required by operating conditions. Power stored in the energy storage system 126 can supplement power from the generator 124 during acceleration.
FIG. 1B shows a block diagram for a typical parallel hybrid, indicated generally at 140. In a parallel hybrid, the engine 122 is connected directly to a continuously variable transmission (“CVT”) 125. The CVT 125 comprises a plurality of electric motor/generators, which both add power to the engine power and facilitate operation of the CVT 125. As with the motor/generator 128 in the series hybrid, these motor/generators also recapture braking energy, and return it to the energy storage system 126.
Parallel hybrids have proved to generally more fuel efficient, because they provide for a continuous mechanical connection between the engine and the wheels. (Some energy is always lost when it is converted from mechanical to electrical, or vise versa.)
An important feature of both types of hybrids is that the wheel speed is decoupled from engine RPM. Consequently, the average operating conditions for the combustion engine in a hybrid are quite different from those for a standard vehicle. This contrasts from most conventional vehicles, in which the wheel speed is coupled to the engine speed through a very limited number of gear ratios. Therefore, the overall fuel efficiency and emissions performance of hybrids can be further improved with corresponding optimization of the combustion engine that exploits that decoupling.
It will also be appreciated that CVTs can be used on a standard (i.e. non-hybrid) vehicle, as well. Consequently, the engines on such vehicles could likewise be improved using similar optimizations. FIG. 1C is a block diagram of a typical standard diesel engine with a CVT, which components correspondingly numbered as in FIGS. 1A and 1B.
Engine performance curves show engine performance characteristics as a function of the engine's revolutions per minute (“RPM”). The performance curves most frequently analyzed include the output power, torque curve, and brake specific fuel consumption curve. These curves show performance at full throttle, “high idle,” (when the operator's foot is pressing the accelerator completely to the floor).
The brake specific fuel consumption curve shows fuel consumption per RPM. Brake specific fuel consumption is typically indicated in units of grams of fuel consumed per BHP per hour (g/(BHP·h)). In general, engine fuel consumption depends greatly on the combustion efficiency and mechanical efficiency. Hybrid vehicles are characterized by excellent fuel consumption because of their ability to recapture braking energy.
The torque curve shows torque as a function of RPM. In general, the less torque changes with RPM, the easier the engine is to handle. The highest torque requires the highest air flow, and thus, in turbocharged engines the intake manifold pressure (“boost”) needs to be at the highest levels. (It will be appreciated that almost all modern heavy duty truck and bus engines are turbocharged.) Producing a very high boost at lower RPM, where the maximum torque (“torque peak”) occurs in a traditional torque curve is limited by the turbocharger's ability to produce that boost while still having some margin from the “surge” line. At the same time, turbomachinery must have enough capacity for the higher airflow required at higher engine speeds (known as sufficient “swallowing” capacity.) Such demands provide various trade-offs in turbomachinery design which have a significant limiting effect on the engine's maximum power and thermal efficiency.
FIG. 2 shows a highly typical torque curve, indicated at 200. The torque curve 200 produces relatively high power at the engine's rated speed 220, but maximum torque occurs at a somewhat lower speed, shown at 230. The difference between the torque at these two points is called the “torque rise,” shown at 210. Typically, the ratio between the RPM at 230 and 220 is around 1.3 to 1.4. This torque curve provides good performance over a range of engine speeds, which is important for vehicles having a limited supply of gear ratios.
The relatively wide plateau of this torque curve permits smooth operation throughout a range of vehicle speeds (and through a range of operating conditions, including changes in incline, wind speed, etc.), both permitting the vehicle to accelerate smoothly up through the transmission's gears, and to cruise within a comfortable range of conditions without requiring frequent gear shifting.
In order to accelerate smoothly, the plateau of the torque curve must be broad enough to cover the transmission's “step,” that is, the ratio between the gear ratios of adjacent gears. In a typical bus or medium truck (e.g. dump trucks, garbage trucks, etc.) the transmission has 8 to 13 gears, and a step of about 35% to 38%. (Passenger cars typically have 4 to 6 gears, with correspondingly higher step, while tractor trucks typically have substantially more gears, with correspondingly lower step.) Thus, when the truck up-shifts, the engine RPMs must be reduced by about 35% while the truck's speed remains constant as the new gear is engaged. The engine must be able to supply sufficient power at the reduced RPM to accelerate the truck, and continue to supply sufficient power until sufficient RPMs are achieved to permit the step down for the next gear shift.
Similarly, the width of the plateau permits the vehicle to make adjustments in cruising speed required by traffic or other road conditions without having to change gears frequently.
FIG. 3 shows a somewhat less common (but not uncommon) torque curve, characterized by the lack of a torque rise. Such a torque curve might be used because of a torque limitation in the transmission system, or for vehicles in which smooth operation throughout a very wide range of RPM is useful (such as dump trucks, cement mixers, etc.).
Likewise, other torque curves are used for other applications, according to the anticipated operating conditions for the vehicle. For example, agricultural tractors typically have torque curves that peak at low RPM. This high torque rise permits them to generate good pulling power from a stop.
The present set of emissions standards for HD diesel engines, as well as HD natural gas engines and a variety of other diesel-derived engines, measures performance at a variety of operating conditions that are defined by the engine's torque curve. One set of these test conditions, known as the “Euro Ill Test Cycle,” is illustrated in FIG. 4. The engine is operated at steady state at each of the 13 indicated points corresponding to various load levels at three engine speeds, plus idle. As shown in FIG. 4, at each of the three engine speeds, “A,” “B,” and “C,” the engine is operated at load levels that require the maximum torque output at that speed, and at 75%, 50%, and 25% of that maximum torque. FIG. 5 shows how the engine speeds “A,” “B,” and “C” are defined. As shown in FIG. 5, the maximum power torque output at any speed is determined. The highest engine speed that produces exactly 70% of that maximum is the so-called “100% speed,” and the lowest engine speed that produces exactly 50% of that power torque is the so-called “0% speed.” Note that, despite their names, these speeds do not correspond to the highest or lowest speeds at which the engine can run, nor to the speeds at which the engine produces 100% or 0% of its maximum power torque. Intermediate speeds are distributed linearly between these two speeds according to engine RPM. Thus, the “A” speed, or 25% speed, is equal to the 0% speed plus 25% of the difference between the 0% speed and the 100% speed. The “B” speed is the 50% speed, and the “C” speed is the 75% speed.
FIG. 6 illustrates another aspect of emissions standards today, called the “Not to Exceed” (“NTE”) zone. The engine's emissions at every operating condition within the NTE zone must be within the specified limits, and is subject to random testing. The NTE zone is bounded on the left by the so-called 15% speed (as defined by the Euro III Test Cycle, described hereinabove), and on the bottom by the greater of 30% of the maximum torque output at any speed and 30% maximum power, as shown in FIG. 6.
FIG. 7 illustrates yet another aspect of emissions standards, called the HD Transient Test Cycle. In this test cycle the engine is run for one second each at 1200 separate conditions selected to immulate typical road conditions. The points illustrated in FIG. 7 are a representative sample of the conditions that might be used in the Transient Test Cycle.
As these test cycles illustrate, prior art engines must meet emissions standards at a wide variety of operating conditions, which requires a substantial amount of design compromise throughout. Thus, in order to meet continuing demand for improved fuel economy and reduced emissions, what is needed is an engine that is tuned specifically for use with a hybrid electric engine. A conventional engine is also need that is tuned for use with a CVT that provides improved fuel efficiency and reduced emissions. The present invention is directed towards these needs, among others.